feat(desktop): manifeste de mise a jour latest.json + builds locaux signes (#77)

- scripts/updater_manifest.py : construit latest.json (version, pub_date, platforms Windows/Linux avec signature .sig et URLs des assets).

- publish_release.py : genere latest.json, upload des .sig + latest.json, rappel de commit.

- tauri.conf.json : endpoint updater -> raw/branch/main/desktop/latest.json.

- build-windows.bat / build-linux.sh : detection automatique de obsigate-updater.key (signature) ou repli non signe.

- .gitignore : ignore desktop/key/ ; tests/test_updater_manifest.py (8 tests).

.gitignore

@@ -34,4 +34,6 @@ backend/VERSION
 # Tauri updater signing keys (private key — never commit)
 desktop/*.key
 desktop/*.key.pub
+desktop/key/
+